Modern semiconductor systems often depend on accurate measurement of the position of a semiconductor substrate. An example of a tool commonly used in semiconductor wafer metrology and inspection is an electron microscope. FIG. 1 depicts an electron beam microscope system 100 of the prior art. An electron optical column 102 focuses an electron beam 101 onto a surface of a wafer 104. Electrons scattered from the wafer 104 are collected to form an image. To facilitate location of defects on different parts of the wafer 104, the wafer is typically processed on a support 105 having a chuck 106 and XY stage 108 for translation of the chuck (and wafer) in X and Y directions more or less parallel to the plane of the wafer 104. Since the electron beam microscope 100 must operate in a vacuum chamber 103, the chuck 106 is typically a high voltage electrostatic chuck. The change in position of the wafer 104 can be measured using an interferometer system 112 measuring off a stage mirror 110. The stage mirror 110 includes highly polished faces 114 oriented perpendicular to the X and/or Y axes. In the interferometer 112 a beam of light 116 (e.g., from a laser) is split by a beamsplitter 118. Part of the light (referred to sometimes as the reference beam) reflects off a fixed mirror 120 back to the beamsplitter 118. Another part of the light (referred to sometimes as the measured beam) reflects off the highly polished face 114 back to the beamsplitter 118. The beamsplitter 118 combines both parts and the combined optical signal strikes a photodetector 119. An interference signal from the photodetector 119 changes in a predictable way as a result of movement of the stage mirror 110.
As the wafer 104 moves in the X and Y the depth of focus of the electron beam 101 may vary as a result of topographical features or tilting of the surface of the wafer 104. To adjust for variations in topography of the wafer surface the support 105 may include a Z stage 122. The Z stage 122 includes a stage plate 124, one or more piezoelectric actuators 126. The wafer chuck 106 is attached to the Z stage plate 124 by compliant mounts 128. High voltage electrical isolators 130 provide electrical insulation between the chuck 106 and the Z stage plate 124. The stage mirror 110 is mounted to the Z stage plate 124 through kinematic mounts 132, e.g., of the sphere and V-groove type, the sphere and cone type and/or the sphere and flat type. In many prior art systems, the wafer surface is analyzed to determine a slope and then the Z stage 122 is moved up or down to level the wafer statically. Such systems can not dynamically adjust the height of the wafer 104 in response to changing wafer topography.
Furthermore, the Z stage 122 carrying the wafer chuck 106 is mechanically separated from the stage mirror 110, which typically is rigidly coupled to the XY stage 108. As a result of this mounting there is a long mechanical path indicated by the dashed line 134 (sometimes referred to as a metrology loop) between the polished surface 114 on the stage mirror 110 and the wafer chuck 106. Due to this long path static and dynamic XY position errors due to relative motion between the wafer chuck 106 and the stage mirror 110 are properly not tracked by the electron beam 101. Instead, these errors are tracked by image computer alignment at a relatively slow bandwidth. These errors include scan to scan errors, intra-scan errors and high frequency (kernel to kernel) errors. In systems such as that shown in FIG. 1, a position sensor 136, e.g., a capacitor gauge, placed proximate the wafer chuck 106 may be used to characterize these errors by measuring a relative displacement ΔX between the chuck 106 and the stage mirror 110.
In alternative prior art designs, e.g., the Mebes Exara, designed by Etec systems, a substrate (e.g., a mask) may be coupled to the stage mirror 110, but is pre-aligned prior to scanning the mask in the X and Y directions and is not dynamically adjusted in the Z direction during the scan. In this design, static pre-alignment of the substrate is mapped prior to the scan and is not dynamically adjusted into the optical focal plane during the scan. Thus, changes in height have to be compensated by some other means.
Prior art attempts to address tracking errors due to relative motion between the substrate chuck 106 and the stage mirror 110 have been limited by the bandwidth of the deflection system, interferometer data rate and data age. These errors may be large enough to case either a false defect detection or loss of inspection sensitivity.
Thus, there is a need in the art, for a substrate support system that overcomes these disadvantages.